FreeBSD includes a modern version of OpenSSL as part of its base system, but doesn't appear to have a pkg-config file, so the PHP configure script can't find the libraries. When compiling PHP on FreeBSD, you should define the OPENSSL_LIBS and OPENSSL_CFLAGS environment variables before running PHP's configure. For FreeBSD 12, the following will work:
export OPENSSL_LIBS="-L/usr -lssl -lcrypto -lz" && export OPENSSL_CFLAGS="-I/usr/include" && ./configure --with-openssl [...other configure options...]
